Baldness is the absence of hair on the head. Baldness frequently becomes a problem as people age. There is generally gradual thinning of the hair over the head. The top of the head may become completely bald.
Baldness may also be hereditary.
While the causes of baldness are not fully understood, changes in hormone levels, such as that of the male hormone testosterone and/or androsterone, are thought to play a part. Infection may also.
Hair consists of a root and a shaft. The root is a soft bulb, while the rest is the shaft. The root and a section of the shaft below the skin surface lie in a follicle sac. The bottom of the follicle sac projects the papilla which contains an artery that nourishes the root.
The hair grows by forming new cells at the base of the root. The cells form around the nourishing papilla, as the old ones are pushed away and die and became part of the shaft. Human scalp hair usually grows one-half inch per month for two to four years. Then the shaft falls off and a new shaft replaces it. When the old shaft falls off, the papilla becomes active again and new hair appears.
Baldness results when the old shaft is no longer replaced.